Resistors control the flow of current. Each resistor is a load on the circuit, so it will have a voltage drop.
More resistance results in less current because it is more difficult for electrons to flow through the circuit.
Theory 1: Resistors
1. Which resistor produced the brightest LED?
2. Which resistor draws the most current? (check the I value on Falstad)
3. What happens to the current when the resistance is decreased?
4. Which circuit has the largest voltage drop?